Mapping the N83 region in the SMC

The Small Magellanic Cloud (SMC) is the second closest irregular galaxy to the Sun, located at a distanceof 60 kpc. It provides a highly interesting site to investigate star formation under different conditions thanthose present in our own Galaxy - the metalicity is approximately 10 times lower than the Solar value, andthe ratio of gas to dust is 17 times higher than our Galaxy. The star formation rate in the SMC is high andit contains several active star forming regions with associated molecular clouds and HII regions. The com-bined effect of increased star formation and higher gas to dust ratios lead to increased UV fluxes and hencelower molecular abundances than in the Milky Way. The main scientific questions regarding the SMC are re-lated to understanding these very different environmental conditions and their effect of the formation of stars.
The molecular gas in the SMC has been studied in detail using the J=1-0 and J=2-1 transitions ofCO as part of an ESO-SEST key program (e.g. Rubio et al. 1993, Rubio et al. 1996). Molecular emissionoccurs in 3 main region; the NE Bar, the SW Bar and the Wing. The large scale distribution of CO in theseregions has been observed using the NANTEN telescope (spatial resolution 2.6') showing that the emissionis concentrated around the main star forming regions (Mizuno et al. 2001). This indicates that the COclouds are rapidly dispersed after the formation of star clusters and associations. Studies of the mid tofar infrared emission with IRAS and ISO show that most of the detected sources are associated with themolecular clouds and are cold (T < 30 K) (Wilke et al. 2003).
The two brightest CO clouds in the SMC are located in the SW Bar and have already been mapped inCO(3-2) and observed in several molecules using the SEST (Chin et al. 1998 in LIRS36 and Heikkil¨ et al.a1999 in LIRS49). The LIRS36 cloud has also been observed in CO(4-3) during the APEX commissioningperiod. The next brightest CO complex is the N83/N84 region in the SMC Wing. This has been studiedin detail using CO(1-0) and (2-1) by Bolatto et al. (2003) but has not been observed in other transitions.Maps in CO(2-1) of all these sources indicate that they have a complex structure that is not fully resolvedat the angular resolution of the SEST (Israel et al. 2003).
The N83 region would make a particularly interesting target for mapping during APEX science verificationfor several reasons:
· It is one of the few isolated, but relatively active star-forming regions in the Wing region (which is otherwise inconspicuous in CO emission).
· The main molecular cloud in the complex appears to be interacting with an expanding shell that has been suggested to be a supernova remnant.
· The spatial resolution of an APEX-2A CO(3-2) map would match very well with the existing (2-1) map of Bolatto et al. (2003).
· Several regions of unusually high (2-1)/(1-0) ratios were found in the complex, one of which was associated with the centre of the possible SNR expanding shell.
· If this is indeed an expanding supernova remnant, the chemistry and excitation in the cloud could be dominated by the SNR shock.
During the science verification we will not have time to repeat a full molecular study of N83/N84 asalready carried out for LIRS36 and LIRS49 because observing the emission from molecules other than COrequires very long integration time. However, mapping the main part of the cloud (N83B and N83C) inCO(3-2) will allow us to improve upon the previous estimate for the CO-H2 conversion factor and char-acterise the excitation conditions, particularly along the edge of the cloud that bounds the possible SNRshell. If the FLASH receiver is available, we could then map the central part of the N83 cloud in CO(4-3),allowing a more detailed investigation of the excitation in the central region. Several pointed observationsof 13 CO(3-2) would allow us to determine the optical depth in the 12 CO lines.
Previous observations of CO(2-1) show peak main beam brightness temperatures of 2.7 K towards theCO peak corresponding to N83C. The (3-2) observations in LIRS36 and LIRS49 show that the ratio (3-2)/(2-1) is around, or just below 1. Assuming a beam efficiency of 0.7, this would give peak (3-2) antennatemperatures in N83B/C of 1.5-1.7 K. The lines widths are approximately 3 km s-1 and so a bandwidth of1024 MHz and 2048 channels will be sufficient to detect the lines. With half-beam spacing of 9", the mainsource including the interface with the SNR could be mapped with a grid of 11×17 positions (90"×148"). Araster map with 3 ONs per OFF and 15 seconds integration per point would give an RMS of 0.13 K and totaltime of 62 minutes (assuming a system temperature for APEX-2A of 250 K). Previous observations duringcommissioning indicate that overheads (including calibration and pointing observations) would increase thistime by a factor of 3 to 3.1 hours.
In order to calculate the optical depth of the 12 CO lines, we would need pointed measurements of13CO(3-2). The 12CO/13CO intensity ratio in the SMC is generally found to be of the order 10. For 2pointed observations towards the two peaks in the cloud (N83B and N83C), we would require 2.2 hours(including overheads).
If the FLASH receiver is available, it would enable a much better analysis of the excitation in the cloudto observe CO(4-3) in the central region. The measured antenna temperatures from the LIRS36 cloud showthat (4-3) should not be much weaker than (3-2). Assuming a peak antenna temperature of 1.5 K andsystem temperature of 1000 K, would require 4.3 hours to reach an RMS of 0.2 K in a 5×5 grid (with 1ON per OFF).
In the previous studies of Bolatto et al. (2003), unusually high (2-1)/(1-0) ratios (>2) were found atthe centre of the expanding (?SNR) shell. It would be extremely interesting to probe this region further anddetermine the (3-2)/(2-1) ratios. CO(2-1) main beam temperatures in this region are 0.5 K. Assumingthat CO(3-2) has a similar brightness to (2-1) in this region, we would be able to reliably detect it with anRMS of 0.04 K. Mapping over a 3×3 grid would require 2.4 hours. If the emission is easily detected, thismap could be extended over a wider area in the same time.
In total this project would require 12 hours of telescope time. In July 2005, the SMC reaches a peakelevation of 40 degrees at LST 01:00. It is observable above 30 degrees for approximately 8 hours during thelater part of the night and sun rise. Therefore, we would need to split the observations over 2 nights.